Optimize system note service specs

What does this MR do?

Optimize system note service specs.

Before:

Finished in 1 minute 26.13 seconds (files took 20.05 seconds to load)
330 examples, 0 failures
["391 ./spec/services/system_note_service_spec.rb"]

After:

Finished in 49.93 seconds (files took 16.61 seconds to load)
330 examples, 0 failures
["75 ./spec/services/system_note_service_spec.rb"]

The last line is number of INSERT INTO "projects" statements invoked during tests.

What are the relevant issue numbers?

See #37234